Studies using various inhibitors and blocking antibodies have shown that cell mediated cytotoxicity (CMC) by natural killer (NK) cells proceeds in distinct steps--namely recognition or binding of NK to target, programming for lysis and lymphocyte independent target cell lysis. These results suggested that the various steps may be the result of distinct molecular interactions. Our recent studies have demonstrated for the first time the presence of a soluble cytotoxic factor that may play a role in NKCMC. This soluble natural killer cytotoxic factor(s) (NKCF) is derived from natural killer cells and lyse natural killer sensitive target cells. Our results have also shown a good correlation between the known characteristics of NKCMC and the characteristics of NKCF. Therefore, we will (a)\characterize biochemically the nature of the cytotoxic factor, (b)\demonstrate directly its role in NKCMC by the use of antibodies directed against NKCF, and (c)\investigate the role of NKCF receptors on target cells and their relationship to target cell susceptibility to lysis by NK. Methods for the purification and characterization of NKCF will include gel filtration, PAGE electrophoresis, HPLC, and affinity chromatography. In addition, peptide maps and limited sequence analysis will be performed. We will also generate xeno and monoclonal antibodies against NKCF. Anti-NKCF antibodies will be used to determine the role of NKCF in NKCMC. Binding assays of radiolabeled NKCF on NK resistant and NK sensitive target cells will correlate receptor expression with sensitivity of target cell to lysis. The studies are significant because they provide a novel mechanism to investigate NKCMC. The studies are also important because they can be used to explain several clinical or genetic manifestations resulting in NK deficiency.